Incubator cost depends on how big and how automated you want it. And how good you are at using tools and scrounging. High end is over US$1000. Low end is somewhere in the US$0-50 range.
Most chicken egg incubators have features that are unnecessary, like egg turners, or unsatisfactory, like thermostats that can't be set low enough for snake eggs.
The simplest, cheapest, styrofoam Hovabators can be satisfactory for snake eggs if there is enough height. I keep snake eggs inside a closed box, and that box would have to go inside the incubator. If the box touched the heat element, there would be toxic fumes and a fire hazard.
I think that a styrofoam shipping box from a fish store would make a decent incubator. It would need a heating pad, a platform for the egg box(es) and a thermostat, of course. One of these days I should break down and knock one together just to see how much it would cost to make and how well it would work.